This invention relates generally to the conversion of oxygenates to olefins and, more particularly, to light olefins with enhanced carbonyl and, in particular, acetaldehyde, removal or recovery.
A major portion of the worldwide petrochemical industry is concerned with the production of light olefin materials and their subsequent use in the production of numerous important chemical products via polymerization, oligomerization, alkylation and the like well-known chemical reactions. Light olefins include ethylene, propylene and mixtures thereof. These light olefins are essential building blocks for the modern petrochemical and chemical industries. A major source for these materials in present day refining is the steam cracking of petroleum feeds. For various reasons including geographical, economic, political and diminished supply considerations, the art has long sought a source other than petroleum for the massive quantities of raw materials that are needed to supply the demand for these light olefin materials.
The search for alternative materials for light olefin production has led to the use of oxygenates such as alcohols and, more particularly, to the use of methanol, ethanol, and higher alcohols or their derivatives such as dimethyl ether, diethyl ether, etc., for example. Molecular sieves such as microporous crystalline zeolite and non-zeolitic catalysts, particularly silicoaluminophosphates (SAPO), are known to promote the conversion of oxygenates to hydrocarbon mixtures, particularly hydrocarbon mixtures composed largely of light olefins.
Such processing of oxygenates to form light olefins is commonly referred to as a methanol-to-olefin (MTO) process, as methanol alone or together with other oxygenate materials such as dimethyl ether (DME) is typically an oxygenate material most commonly employed therein. Such processing typically produces or results in a range of olefin reaction products as well as unreacted oxygenates and other trace oxygenates. Typical or common MTO processing schemes include an oxygenate absorber whereby circulated water is used to absorb oxygenates, e.g., methanol and DME, from the light olefin product. This oxygenate-containing circulated water is subsequently stripped in an oxygenate stripper to recover methanol and DME, with such recovered materials ultimately recycled to the oxygenate conversion reactor. The dewatered oxygenate conversion product stream resulting from the oxygenate absorber is passed to a CO2 removal zone wherein the dewatered oxygenate conversion product stream is contacted with caustic to remove carbon dioxide and produce a caustic treated reactor product stream such as for subsequent processing through an appropriate light olefins recovery system.
Carbonyls, such as acetaldehyde, are common trace oxygenates in the oxygenate conversion reactor effluent and will typically be absorbed in the circulated water. Acetaldehyde, however, is commonly only incompletely stripped in the following oxygenate stripper such that the circulated water may experience a build-up in acetaldehyde concentration. The build-up of acetaldehyde and other carbonyls in the circulated water may severely decrease the effectiveness of the oxygenate absorber for removing acetaldehyde and other carbonyls. Incomplete removal of acetaldehyde and carbonyls may result in contamination of the treated olefin products. Moreover, acetaldehyde is known to cause fouling in the caustic scrubber positioned downstream of the oxygenate absorber.
Aqueous bisulfite solutions are known to react with aldehydes and other carbonyls, preferably methyl substituted carbonyls, to form a bisulfite addition product. As long as unreacted bisulfite ion is present, the bisulfite addition product will form. Sulfite, bisulfite, and the bisulfite addition products are typically either nonvolatile or have a sufficiently low volatility so as to avoid the significant removal thereof upon stripping associated aqueous solutions.
Aldehydes in MTO effluent may, for example, include formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, and crotonaldehyde. These compounds may be in the MTO reactor feed, created as reaction side products, or formed in processing downstream of the reactor.
Aqueous sulfite solutions generally contain an equilibrium mixture of bisulfite and sulfite ions. As the pH of such solutions is lowered below 7.0, bisulfite becomes the predominate species. A pH below 7.0 is generally preferred so as to have sufficient bisulfite present to react with the carbonyls. A pH above 6.5 is generally preferred so as to minimize pitting of carbon steel equipment.
In view of the above, there is a need and a demand for improved processing and systems for the conversion of oxygenates to olefins and, more particularly, for such processing and systems such as to enhance the removal, recovery or separation of carbonyls such as acetaldehyde, such as to facilitate or otherwise improve downstream processing.